In a falling-film evaporator it is essential that the liquid to be evaporated be distributed as uniformly as possible to a multiplicity of generally upright heating tubes disposed between an upper tube sheet or plate and a lower tube sheet or plate, the tubes generally having their upper and lower ends flush with the upper and lower surfaces of the upper and lower tube sheets, respectively. The uniform distribution becomes increasingly important as the concentration of the liquid increases and the evaporation approaches the final concentration.
While it has been proposed heretofore to provide perforated plates or sieve plates above the upper tube sheet in order to distribute the materials of the evaporator to the heating tubes more uniformly, such perforated or sieve plates have been found to give only a coarse distribution of the material to be evaporated, particularly when the properties, e.g. viscosity, temperature or solids content of the material, vary.
It has also been proposed to provide triangular notches in heating tubes which protrude above the upper tube plate (H. Satone, CHEMICAL ECONOMY AND ENGINEERING REVIEW, Vol. 6, No. 5 (1974), pages 20 to 25). These notches have not been found to be successful because the pressure drop from the inlet of the evaporator to the inlet of the heating tubes varies with varying space velocities.
Elsewhere the liquid has also been distributed by means of nozzles having conical bodies disposed within the tubes. These nozzles have been found to give satisfactory results at high supply rates but have not been found to be satisfactory with high concentrations of the material to be evaporated. Furthermore, these arrangements give rise to additional energy losses because the pressure of the material to be evaporated must be increased to the value required by the nozzle having the conical filler bodies.
In printed German application (Auslegeschrift) No. 15 19 741, material to be evaporated is retained in a cup which is provided above the tube plate and the material is distributed through holes onto the upper tube sheet. In spite of the fact that this arrangement allows the material to be evaporated to be uniformly supplied to various portions of the upper tube sheet, it has been found that this arrangement does not guarantee that the material will also enter the heating tubes at equal rates. In fact, it does not do so when the material to be evaporated has entered the evaporator at a temperature above the evaporation temperature (i.e. when a supersaturated or elevated pressure condition) because vapor tends to flash off and will flow transversely to the distributed material as it flows out of the cup. The material leaving the cup, together with the material on the tube plate, is displaced toward the center of the latter and nonuniform distributions are found.
It is also known to provide the inlets of the heating tubes of a falling-film evaporator with inserts which restrict the cross section of flow so that the material to be evaporated can be retained above the openings to facilitate uniform distribution (H. Satone, CHEMICAL ECONOMY AND ENGINEERING REVIEW, Volume 6, No. 5 (1974), pages 20 through 25).
It has been found, with such systems, that uniform distribution is not ensured unless the material to be evaporated is at a temperature below the evaporation temperature. At temperatures above the evaporation temperatures, there is a correspondingly high variable pressure which renders ineffective throttling of the flow of liquid through openings having a predetermined cross section as to uniformity of distribution in feed of liquids into the interior of the heating tubes.